Whatis Down'ssyndrome?Give itssymptoms and cause.Why is it that the chances of having a child with Down'ssyndrome increases if the age of the mother exceedsforty years?

During a medical investigation an infant was found to possess an extra chromosome 21 . Describe the symptoms the child is likely to develop later in life. Why in females exceeding 40-45 years of age are children with Down's syndrome more likely to be born.